Different Stress Tolerances of Juveniles of the Coral <i>Acropora tenuis</i> Associated with Clades C1 and D <i>Symbiodinium</i>.

Abstract

<b>Ikuko Yuyama, Takashi Nakamura, Tomihiko Higuchi, and Michio Hidaka (2016)</b> Reef-building corals are often associated with multiple clades of symbiotic dinoflagellate <i>Symbiodinium</i> spp., where the relative composition of <i>Symbiodinium</i> can alter the phylogenetic properties (<i>e.g.</i>, stress responsiveness, growth rate) of the host coral. The genus <i>Symbiodinium</i> contains nine clades, some of which behave differently in response to strong light and/or temperature stresses, for example, clade D <i>Symbiodinium</i> are thermally tolerant. However, previous studies are based on corals present in the field, and it is possible that the corals used in previous experiments did not contain single <i>Symbiodinium</i> clades. For an accurate assessment of the effects of each <i>Symbiodinium</i> clade on host thermal stress resistance, monoclonal cultures of clades C1 and D were inoculated into aposymbiotic juvenile polyps. Photosynthetic efficiency (maximum quantum yield: <i>F</i> <i>_v</i> <i>/F</i> <i>m</i>) showed a decline at 30°C than at 25°C in both clades. <i>Symbiodinium</i> clade C1 showed a consistently higher rETRmax with larger fluctuations than clade D, with a lower survival rate of juveniles during thermal stress treatment. Under strong light exposure, corals containing clade C1 showed a greater decline in <i>F</i> <i>_v</i> <i>/F</i> <i>m</i> (-74%), compared to decline in corals associated with clade D (-50%) after 3 hours. This is the first study to assess stress tolerances of juvenile corals in association with the monoclonal <i>Symbiodinium</i> clades C and D, and our results indicated greater tolerance of corals associated with clade D to strong light (500 μmol m^-2 s^-1). However, it is difficult to determine the impact of high-temperature stress on coral-algae symbiosis from photosynthetic activity. At high temperatures, clade C1 <i>Symbiodinium</i> exhibited high photosynthetic activity, but host survival rates were higher in corals associated with clade D <i>Symbiodinium</i>. Since clade C1 has a relatively high photosynthetic activity under high temperatures, clade C1 symbiosis at high temperatures might have a negative impact on corals compared with clade D.